U.S. patent application number 09/466865 was filed with the patent office on 2003-08-21 for method for detecting or monitoring sulfur dioxide with an electrochemical sensor.
This patent application is currently assigned to Hathaway Brown School. Invention is credited to DUDIK, LAURIE A., LAI, ANN, LIU, CHUNG-CHIUN.
Application Number | 20030155241 09/466865 |
Document ID | / |
Family ID | 27736996 |
Filed Date | 2003-08-21 |
United States Patent
Application |
20030155241 |
Kind Code |
A1 |
LAI, ANN ; et al. |
August 21, 2003 |
METHOD FOR DETECTING OR MONITORING SULFUR DIOXIDE WITH AN
ELECTROCHEMICAL SENSOR
Abstract
A thick film electrochemical micro-sensor device for detecting
or monitoring sulfur dioxide, comprising a substrate to which is
applied a working electrode, a counter electrode, a reference
electrode, and optionally a heater and a temperature detector,
wherein a portion of the electrodes is covered with an insulator,
and a portion of the electrodes is covered with an electrolyte. The
device is especially useful for detecting or monitoring sulfur
dioxide in emission gases. A method of detecting or monitoring
sulfur dioxide emissions using the electrochemical micro-sensor
device includes contacting the emission gas with the sensor of the
present invention, measuring the current output of the sensor,
determining if the current output indicates the presence of sulfur
dioxide, and generating a signal, that can be used to actuate a
scrubber system when a pre-determined level of sulfur dioxide is
detected.
Inventors: |
LAI, ANN; (BEACHWOOD,
OH) ; DUDIK, LAURIE A.; (SOUTH EUCLID, OH) ;
LIU, CHUNG-CHIUN; (CLEVELAND HEIGHTS, OH) |
Correspondence
Address: |
JOSEPH G CURATOLO, ESQ.
RENNER KENNER GREIVE BOBAK TAYLOR & WEBER
24500 CENTER RIDGE ROAD, SUITE 280
WESTLAKE
OH
44145
US
|
Assignee: |
Hathaway Brown School
19600 North Park Blvd.
Shaker Heights
OH
44122
|
Family ID: |
27736996 |
Appl. No.: |
09/466865 |
Filed: |
December 17, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60166955 |
Nov 22, 1999 |
|
|
|
Current U.S.
Class: |
204/461 ;
204/603; 204/612 |
Current CPC
Class: |
Y02A 50/248 20180101;
Y02A 50/20 20180101; G01N 33/0042 20130101; G01N 27/4045
20130101 |
Class at
Publication: |
204/461 ;
204/603; 204/612 |
International
Class: |
G01N 027/26; C02F
001/469; B01D 057/02 |
Claims
1. An electrochemical micro-sensor device for detecting or
monitoring sulfur dioxide comprising a substrate supporting an
arrangement of a working electrode, a reference electrode, and a
counter electrode, wherein a first portion of the electrodes is
covered with an insulator, and a second portion of the electrodes
is covered with an electrolyte, and wherein the electrodes and the
insulator are applied to the substrate using a thick film
technique.
2. The electrochemical micro-sensor device of claim 1, wherein the
substrate is an insulating material selected from the group
consisting of plastic, glass, ceramic, quartz, and mixtures
thereof.
3. The electrochemical micro-sensor device of claim 1, wherein the
substrate is alumina.
4. The electrochemical micro-sensor device of claim 1, wherein the
working electrode and counter electrode are each independently
selected from the group consisting of gold, platinum, palladium,
silver, silver-silver chloride, carbon, and mixtures thereof.
5. The electrochemical micro-sensor device of claim 1, wherein the
reference electrode comprises one of silver-silver chloride and
mercury-mercuric chloride.
6. The electrochemical micro-sensor device of claim 1, wherein said
first portion of the electrodes is a connect portion and said
second portion of the electrodes is a sensing portion, and wherein
the connect portion connects the electrode to an electrical
circuit, and is protected from the environment by the insulator,
and wherein the sensing portion is exposed to the environment via
the electrolyte.
7. The electrochemical micro-sensor device of claim 1, wherein the
electrolyte comprises an ion conductive resin or membrane.
8. The electrochemical micro-sensor device of claim 1, wherein the
thick film technique comprises the steps of: providing at least one
template containing a pattern for the arrangement of the three
electrodes; contacting the substrate with the template; applying at
least one electrode precursor ink, and insulator precursor ink onto
the template/substrate to form a sensor configuration according to
the template pattern; drying the sensor configuration; firing the
sensor configuration; and covering a portion of the three
electrodes with an electrolyte.
9. The electrochemical micro-sensor device of claim 1, further
comprising a temperature detector.
10. The electrochemical micro-sensor device of claim 9, wherein the
temperature detector comprises platinum.
11. The electrochemical micro-sensor device of claim 1, further
comprising a heater.
12. The electrochemical micro-sensor device of claim 11, wherein
the heater comprises a substantially serpentine pattern of
conductive material printed onto the opposite side of the substrate
from the three electrodes.
13. The electrochemical micro-sensor device of claim 1, wherein the
working and counter electrodes are disposed adjacent to each other,
with a gap therebetween of less than or equal to about 0.2 inches
over at least 90 percent of their length.
14. The electrochemical micro-sensor device of claim 1, wherein the
arrangement of three electrodes is a substantially elliptical
arrangement wherein the working and counter electrodes are
substantially concentrically oriented with respect to each other
without the reference electrode interposed between them.
15. The electrochemical micro-sensor device of claim 1, wherein the
arrangement of three electrodes is a substantially circular
arrangement wherein the working and counter electrodes are
substantially concentrically oriented with respect to each other
without the reference electrode interposed between them.
16. The electrochemical micro-sensor device of claim 1, wherein the
arrangement of three electrodes is a rectangular arrangement
wherein the working and counter electrodes are substantially
concentrically oriented with respect to each other without the
reference electrode interposed between them.
17. The electrochemical micro-sensor device of claim 1, wherein the
arrangement of the three electrodes includes an adjacent working
electrode and counter electrode, wherein portions of the working
electrode are interdigitated with portions of the counter
electrode, and wherein the reference electrode is disposed
outwardly from the working and counter electrodes.
18. A method of detecting or monitoring sulfur dioxide in an
emission gas comprising contacting the emission gas with the sensor
of claim 1; measuring the current output of the sensor; determining
if the current output indicates the presence of sulfur dioxide; and
generating a signal.
19. The method of claim 18, further comprising transmitting the
signal to at least one device selected from the group consisting of
display devices, recording means, alarm devices, and compensating
means.
20. The method of claim 19, wherein the compensating means
comprises a scrubber system, a diversionary means, a trapping and
condensing means, or a combination thereof.
21. The method of claim 18, wherein the sensor is maintained at a
constant temperature higher than the temperature of the emission
gas contacting the sensor.
22. The method of claim 18, wherein said step of determining if the
current output indicates the presence of sulfur dioxide comprises:
generating a first signal based on the current output of the
sensor; providing at least a second sensor substantially identical
to the sensor, wherein the second sensor is adapted to detect
interference from other chemical species besides sulfur dioxide;
contacting the emission gas with the second sensor; measuring the
current output of the second sensor; generating a second signal
based on the current output of the second sensor; and subtracting
the second signal from the first signal.
23. The electrochemical micro-sensor device of claim 1, wherein the
insulator is a second insulating material selected from the group
consisting of glass, and a glass-containing dielectric material.
Description
[0001] The present application claims priority under 35 U.S.C.
.sctn.119(e) from U.S. Provisional Patent Application No.
60/166955, filed on Nov. 22, 1999.
TECHNICAL FIELD
[0002] The present invention is directed to an electrochemical
micro-sensor device for detecting or monitoring sulfur dioxide.
More particularly, the invention is directed to a thick film
electrochemical micro-sensor device capable of detecting and
monitoring sulfur dioxide emissions.
BACKGROUND OF THE INVENTION
[0003] Acidic deposition, which includes acid rain, acid snow, and
even acidic dusts, is currently a major environmental issue. It is
not only a critical threat to the natural world but also to the
world of man. In recent years, acidic deposition has increased
greatly which, in turn, amplifies its effects. It has depleted the
fisheries in various lakes around the world. It has also caused the
destruction of great forests around the world, especially in
Germany. Moreover, acidic deposition is, in itself, damaging human
health in various locations. Finally, acidic deposition poses a
great threat to the preservation of human history. All these
problems will be discussed more thoroughly in the following
paragraphs.
[0004] First of all, acidic deposition is a main factor behind the
depletion of aquatic life in various lakes whether natural or
man-made. When acidic deposition occurs, the acidic chemicals mix
with the water in the lakes, gradually increasing the acidity of
the lakes. Because all species of aquatic life forms can only
reproduce and certain eggs can only hatch at certain acidic levels,
these species are unable to reproduce regularly if they can
reproduce at all. This is a major concern for various fish species
as well as certain small shrimps and mollusks. Furthermore, the
acidic chemicals from repeated acidic deposition leaches dangerous
metals, such as mercury and lead, from the soil at the lake bottoms
and into the lake waters. Eventually, the concentration of these
toxic metal substances in the lake reach a certain level where the
organisms living within the lake waters start dying off specie by
specie. After a certain time, these acidified lakes will look
crystal clear, but that will be because there will be nothing
living in the lakes except maybe a couple of algae species.
[0005] Acidic deposition is also a main cause behind the
devastation of major forests around the world, especially in
Germany. The acidic deposition descends into the forests and
permeates through the soil. Similar to its effects on the lakes,
the acidic chemicals once again release dangerous chemicals, such
as aluminum, except this time from the soil. These metals instigate
a slow process which kills the trees. Another effect of acidic
deposition is that the acidic chemicals, once in the soil, displace
the nutrients, such as calcium, which are important to trees and
other plant life, from the ground. Because of this displacement,
the plant life in such forests is unable to absorb the nutrients
necessary for its survival from the soil at forest bottoms.
[0006] Acidic deposition is not only an environmental problem but
also a problem for humans because it is damaging to human health as
well. First of all, the acidic chemicals from acidic deposition,
especially from acid rain and acid snow, have the ability to leach
toxic metals, such as copper and lead, into drinking water for
humans. The presence of these metals in everyday drinking water
would obviously harm the health of humans who intake this water.
Moreover, the acidic chemicals have been found to be the main
sources behind the outbreaks of gastroenteritis in places such as
the Adirondack Mountains. Furthermore, when in high concentrations,
the acids are capable of irritating the human respiratory system
causing problems in everyday functions such as breathing. Finally,
medical institutions have also suspected that the acidic content of
rain, snow, and dust may be the cause of some types of chronic
bronchitis and emphysema which would eventually lead to chronic
heart disease.
[0007] Lastly, another major problem initiated by acid rain is the
destruction of human history by the deterioration of man-made
historical architectures. History buildings, such as the
magnificent Acropolis in Greece and the stunning Pantheon in Rome,
have survived the weathering from rain and snow and wind for
hundreds of centuries. However, in recent years, with the rapid
increase in acidity in rain, snow, and even dust, these famous
buildings representing the progress and history of mankind have
deteriorated at higher and higher rates. This deterioration is an
obvious effect of the corrosive acidic content of the rain. Because
these structures are made mostly in limestone and marble which are
basic, the acid reacts with the stones and corrodes the surfaces.
However, the deterioration not only occurs with stone monuments and
buildings, but also affects the metal structures by increasing the
rate of oxidation or rust of the metal. Therefore, acid deposition
not only plays a significant part in the destruction of the
environment but it also affects humans through damaging human
health and deteriorating the traces of stepping stones in human
history.
[0008] There are three main types of acidic deposition. Acidic dust
or ashes are acidic chemicals that descend from the sky in the form
of dry flaky solids. Another type of acidic deposition is acidic
snow or sleet. This type of acidic deposition occurs with the
freezing or crystallization of the acid rain while or before it
precipitates from the atmosphere. Finally, most importantly, there
is acid rain. Acid rain occurs when the acidic compounds released
in gaseous form make contact with the moisture in the atmosphere.
Then, when the moisture reaches a certain level and descends from
the sky, the acidic compounds have already combined with the rain
droplets making the rain acidic.
[0009] There are two main groups of gases that contribute to the
formation of acidic deposition. The first group includes the group
of nitrogen oxides. Its main contributors are automobiles. Another,
which is the major contributor to acidic deposition, is sulfur
dioxide which is released mainly from the smokestacks of industrial
buildings. Outside of these two groups, there are some other minor
groups of gases that also contribute to acidic deposition, such as
carbon dioxide. When these various acidic gases are released into
the atmosphere, they come in contact with the moisture in the
atmosphere. Then, with the water, the gases form acidic solutions
which fall to the ground in the form of acid rain, acid snow, or
acidic dust as described in the previous paragraph.
[0010] As mentioned previously, the main contributor to acidic
deposition is sulfur dioxide. Sulfur dioxide is usually produced as
a result of burning coal and oil. It is also often produced as a
superfluous product by the processes in refineries, pulp and paper
mills, as well as metal smelters. When released into the
atmosphere, sulfur dioxide and other oxides of sulfur have the
ability to form sulfates with the oxygen as well as aerosols of
sulfurous and sulfuric acids with water vapor. All these chemicals
eventually come down to the ground in various forms of acidic
deposition. Therefore, sulfur dioxide harms the environment as well
as people in the form of acidic deposition. However, in high
concentrations, the sulfur dioxide, by itself, is also a problem.
It, by itself, possesses the ability to aggravate already present
respiratory and cardiovascular diseases including asthma,
bronchitis, and emphysema.
[0011] Therefore, in order to help in reducing acid rain, the
amount of sulfur dioxide released needs to be reduced. Although
there are currently processes such as the use of scrubbers or
calcium carbonate released into the sulfur dioxide emissions to
neutralize the gas, the processes are expensive, and they also
cannot monitor the sulfur dioxide levels. Therefore, it is believed
that the most effective way to reduce sulfur dioxide would be
through monitoring the sulfur dioxide released from industrial
smokestacks. This could lead to a more effective way to regulate
sulfur dioxide emission levels. There are many ways to measure
sulfur dioxide, and one of these ways is through using our newly
developed micro-sensor and sensor technology.
[0012] There are many types of sensor technology used for detecting
gaseous sulfur dioxide. These sensors can determine the presence of
and, usually, the amount of sulfur dioxide in an environment. Of
these sensors, spectrophotometric analyzers, conductometric
sensors, solid electrolyte electrochemical cells, piezoelectric
crystal detectors, and interdigital capacitors (IDCs) are more
commonly used. Each type along with its advantages and
disadvantages will be discussed in the following paragraphs.
[0013] Spectrophotometric analysis is currently the standard
technology used for the detection of sulfur dioxide.
Spectrophotometric analyzers use either ultraviolet or infrared
light to determine the presence of as well as the amount of sulfur
dioxide present. Although spectrophotometric analyzers may be the
standard method of detection, they often suffer from the
interfering absorptions from other sources. Moreover, on top of the
fact that they are very expensive commercially to manufacture, and
the operation of the analyzers is elaborate as well as complicated
in that these analyzers contain mechanical moving parts.
[0014] Another method of sensing sulfur dioxide is through the
application of conductometric sensors. In conductometric sensors,
the sulfur dioxide gas diffuses through a gas-permeable membrane
and reaches equilibration in a layer of water. Within the thin
layer of water, conductometric sensors are positioned. When the
sulfur dioxide gas reaches equilibration in the thin water layer,
these conductometric electrodes determine the conductance of the
sulfur dioxide gas. Using the conductivity measured, the sulfur
dioxide level can be determined in ninety to one hundred and twenty
seconds. The advantages of these conductometric sensors include
high sensitivity to sulfur dioxide. Moreover, they are simpler to
operate than the spectrophotometric analyzers. However, they also
have their disadvantages, which include being non-specific, meaning
that it is difficult for this kind of sensor to differentiate
sulfur dioxide from other gases. Furthermore, another disadvantage
is their constant need for extensive maintenance in comparison to
the maintenance needs of other methods.
[0015] Another method for detection of sulfur dioxide employs solid
electrolyte electrochemical cells. These cells contain a
Na.sub.2SO.sub.4--Li.sub.2SO.sub.4--Y.sub.2(SO.sub.4).sub.3--SO.sub.3
solid electrolyte and a solid reference electrode. These cells
determine sulfur dioxide levels through measuring the electromotive
force (EMF). They are more compact and less expensive than
spectrophotometric analyzers. They are also capable of detecting
sulfur dioxide continuously. Moreover, they have the capability to
respond only to sulfur dioxide gas even in the parts per billion
(ppb) range. However, these solid electrolyte sensors have
disadvantages as well. Their disadvantages include the fact that
they need a regulated supply of a reference gas mixture containing
both sulfur dioxide and air. Another main disadvantage they possess
is that these sensors can only operate at the most optimum quality
within the restricted temperature range of 783 K and 833K.
[0016] There are also piezoelectric crystal detectors which make up
another sector of sensors used for sulfur dioxide detection. When
sulfur dioxide gas is bubbled through a mercurous nitrate solution,
a mercury displacement reaction occurs producing mercury vapor.
Because of the capability of gold to absorb as well as amalgamate
mercury, a gold-coated piezoelectric crystal is used. When the
mercury vapor reaches the gold-coated piezoelectric crystal, the
crystal detects the mercury vapor because a mercury alloy or
amalgam is formed. By measuring and monitoring the mercury using
the piezoelectric crystal, the concentration of sulfur dioxide in
the sample of air can be determined as well. This method is
advantageous in that it has good sensitivity for sulfur dioxide
because it can detect sulfur dioxide both in the parts per billion
(ppb) and in the parts per million (ppm) ranges. Furthermore, these
detectors also have good selectivity for sulfur dioxide in that
they are capable of distinguishing sulfur dioxide from the other
gases in the ambient air. However, these piezoelectric crystal
detectors also suffer from disadvantages. The main disadvantage is
the concern that they may produce and emit hazardous mercury gas
which can cause serious damage and problems.
[0017] Finally, there is a method of detecting sulfur dioxide which
employs interdigital capacitors or IDCs. These IDCs have organic
absorption centers, which are often constructed using organically
modified silicates. These capacitors are claimed to have good
selectivity for sulfur dioxide, which is an important advantage.
However, there is also one big concern facing this method. Because
this method is relatively new, there have been insufficient
experimental data collected supporting this claim of good
selectivity.
[0018] More recent technology of electrochemical sensors, involves
two main basic types of micro-sensors, thick film and thin film.
Thin film electrochemical sensors apply vapor deposition to produce
the sensing elements, which makes the sensors more expensive to
produce. However, the thickness of these thin film micro-sensors is
only a few microns. The other type of electrochemical micro-sensors
is the technology of thick-film sensors which apply a
silkscreen-like process for printing the sensors, and because of
this, the thickness of these micro-sensors is approximately 0.02
inches, making them thicker than thin film sensors.
[0019] Of these thick film electrochemical sensors, there are two
main branches of configuration. One employs a two-electrode
configuration while the other one applies a three-electrode
configuration that is more accurate because it includes a reference
electrode. A two-electrode configuration combines the counter and
the reference electrode into one electrode while the
three-electrode one separates the two electrodes making the results
more accurate. For both types of sensors, the Gibbs free energy is
first calculated from the oxidation-reduction or redox reaction
that occurs when the substance being detected is at the working
electrode. Then the calculated Gibbs free energy will be used to
determine the necessary potential voltage to apply to the working
and counter electrodes in relation to the voltage used in the
reference electrode to allow the specific redox equation to occur.
This calculated potential voltage is applied to the working and
counter electrodes of the micro-sensor against the EMF
(electromotive force) of the silver-silver-chloride electrode.
Furthermore, the corresponding current produced is measured. After
many tests are run with the sensor under various concentrations,
the results are used to calibrate the sensors by fitting a linear
line of concentrations versus current to the data, which can then
be used in order to quantify the sulfur dioxide being measured.
[0020] This technology of thick film electrochemical micro-sensors
has been used in various fields because of its cost efficient as
well as uncomplicated method of manufacture and use. They have been
used to detected acidity in waters and even for monitoring human
health. For example, they have been used in the project CHIME which
uses this type of sensors for monitoring the actions of the heart,
among which is the rate of heart beats of babies who range from
just born to a couple of years old. However, being a relatively new
technology, thick-film electrochemical micro-sensors have not yet
been applied to either detecting sulfur dioxide or determining the
concentration of sulfur dioxide in gaseous samples.
[0021] It is therefore an object of the present invention to
provide a thick film electrochemical micro-sensor for detecting
sulfur dioxide.
SUMMARY OF THE INVENTION
[0022] The present invention provides an effective and economical
electrochemical micro-sensor device for detecting or monitoring
sulfur dioxide comprising a substrate supporting an arrangement of
a working electrode, a reference electrode, and a counter
electrode, wherein a first portion of the electrodes is covered
with an insulator, and a second portion of the electrodes is
covered with an electrolyte, and wherein the electrodes are applied
to the substrate using a thick film technique.
[0023] The present invention more optimally provides an
electrochemical micro-sensor device for detecting or monitoring
sulfur dioxide comprising a substrate containing an arrangement of
a working electrode, a reference electrode, and a counter
electrode, wherein a first portion of the electrodes is covered
with an insulator, and a second portion of the electrodes is
covered with an electrolyte, wherein the electrodes and the
insulator are applied to the substrate using a thick film
technique, and wherein the sensing portions of the working and
counter electrodes are disposed adjacent to each other, with a gap
therebetween of less than or equal to about 0.2 inches, in an
optimum configuration, as described herein.
[0024] The present invention further provides a method of detecting
sulfur dioxide in an emission gas comprising contacting the
emission gas with the inventive sensor, measuring the current
output of the sensor, determining if the current output indicates
the presence of sulfur dioxide, and generating a signal. This
signal can then be used to activate a display device, a recording
means, an alarm device, and/or a compensating means.
[0025] The present invention additionally provides a method of
detecting sulfur dioxide in an emission gas using at least two
micro-sensor devices in a differential mode of operation. The
method comprises contacting the emission gas with a first inventive
sensor, measuring the current output of the sensor, generating a
first signal based on the current output of the sensor, providing
at least a second inventive sensor, which has been adapted to
detect interference from other chemical species, contacting the
emission gas with the second sensor, measuring the current output
of the second sensor, generating a second signal, and subtracting
the second signal from the first signal. This signal can then be
used to activate a display device, a recording means, an alarm
device, and/or a compensating means.
[0026] It has been found that as the size of the working electrode
of the micro-sensors increases, the sensitivity as well as the
current output increases because the surface area at which the
reaction takes place increases. The efficiency of the micro-sensor
device is thus increased as the surface area of the electrodes
increases. Novel electrode configurations designed to maximize this
effect were tested, and the results are reported herein, along with
the preferred electrode configurations.
[0027] It has further been found that as the gap between the
working and counter electrodes decreases, the sensitivity as well
as the output current increases, theoretically because the
electrons have less resistance when transferring from the counter
electrode to the working electrode. The efficiency of the
micro-sensor device is thus increased as the gap between the
working and counter electrodes decreases. Novel electrode
configurations designed to maximize this effect were tested, and
the results are reported herein, along with the preferred electrode
configurations.
[0028] It has also been found that as the length of the working
electrode and counter electrode adjacent to one another increases,
the sensitivity as well as the current output increases because
there is a greater length of the region where the electrons have
less resistance moving from the working electrode to the counter
electrode. The efficiency of the micro-sensor device is thus
increased as the length of the region where the working electrode
and counter electrode are adjacent to one another increases. Novel
electrode configurations designed to maximize this effect were
tested, and the results are reported herein, along with the
preferred electrode configurations.
[0029] There exists a linear relationship between the current
output and the concentration of the sulfur dioxide because, as the
concentration increases, the amount of electrons transferred
increases as well, contributing to a higher current output. This
linear relationship allows the electrochemical micro-sensor device
of the present invention to detect and quantitatively monitor
sulfur dioxide emissions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1A is a schematic illustration of the design of 11
thick film electrochemical micro-sensors prepared and tested in
accordance with the invention, and designated as Set A.
[0031] FIG. 1B is a schematic illustration of the design of 4 thick
film electrochemical micro-sensors prepared and tested in
accordance with the invention, and designated as Set B.
[0032] FIG. 2A is a schematic illustration of a preferred sensor
configuration, example no. 3, in accordance with the present
invention, employing a substrate, working electrode, counter
electrode, reference electrode, and temperature detector.
[0033] FIG. 2B is a schematic illustration of a second preferred
sensor configuration, example no. 7, in accordance with the present
invention, employing a substrate, working electrode, counter
electrode, reference electrode, and temperature detector.
[0034] FIG. 3 is a schematic illustration of the device used for
testing the sensors of examples nos.1-11 and 14-15.
[0035] FIG. 4 is a schematic illustration of the device used for
testing the sensors of examples nos.12 and 13.
[0036] FIG. 5 is a schematic illustration of the chemical reactions
which occur at the electrodes.
[0037] FIG. 6A is a graphical representation of the current output
for the sensor of example no.1 over a range of voltages from -0.6V
to 1V.
[0038] FIG. 6B is a graphical representation of the current output
for the sensor of example no.1 in relation to SO.sub.2
concentrations from 0 to 2%.
[0039] FIG. 7A is a graphical representation of the current output
for the sensor of example no.2 over a range of voltages from -0.6V
to 1V.
[0040] FIG. 7B is a graphical representation of the current output
for the sensor of example no.2 in relation to SO.sub.2
concentrations from 0 to 2%.
[0041] FIG. 8A is a graphical representation of the current output
for the sensor of example no.3 over a range of voltages from -0.6V
to 1V.
[0042] FIG. 8B is a graphical representation of the current output
for the sensor of example no.3 in relation to SO.sub.2
concentrations from 0 to 2%.
[0043] FIG. 9A is a graphical representation of the current output
for the sensor of example no.4 over a range of voltages from -0.6V
to 1V.
[0044] FIG. 9B is a graphical representation of the current output
for the sensor of example no.4 in relation to SO.sub.2
concentrations from 0 to 2%.
[0045] FIG. 10A is a graphical representation of the current output
for the sensor of example no.5 over a range of voltages from -0.6V
to 1V.
[0046] FIG. 10B is a graphical representation of the current output
for the sensor of example no.5 in relation to SO.sub.2
concentrations from 0 to 2%.
[0047] FIG. 11A is a graphical representation of the current output
for the sensor of example no.6 over a range of voltages from -0.6V
to 1V.
[0048] FIG. 11B is a graphical representation of the current output
for the sensor of example no.6 in relation to SO.sub.2
concentrations from 0 to 2%.
[0049] FIG. 12A is a graphical representation of the current output
for the sensor of example no.7 over a range of voltages from -0.6V
to 1V.
[0050] FIG. 12B is a graphical representation of the current output
for the sensor of example no.7 in relation to SO.sub.2
concentrations from 0 to 2%.
[0051] FIG. 13A is a graphical representation of the current output
for the sensor of example no.8 over a range of voltages from -0.6V
to 1V.
[0052] FIG. 13B is a graphical representation of the current output
for the sensor of example no.8 in relation to SO.sub.2
concentrations from 0 to 2%.
[0053] FIG. 14A is a graphical representation of the current output
for the sensor of example no.9 over a range of voltages from -0.6V
to 1V.
[0054] FIG. 14B is a graphical representation of the current output
for the sensor of example no.9 in relation to SO.sub.2
concentrations from 0 to 2%.
[0055] FIG. 15A is a graphical representation of the current output
for the sensor of example no.10 over a range of voltages from -0.6V
to 1V.
[0056] FIG. 15B is a graphical representation of the current output
for the sensor of example no.10 in relation to SO.sub.2
concentrations from 0 to 2%.
[0057] FIG. 16A is a graphical representation of the current output
for the sensor of example no.11 over a range of voltages from -0.6V
to 1V.
[0058] FIG. 16B is a graphical representation of the current output
for the sensor of example no.11 in relation to SO.sub.2
concentrations from 0 to 2%.
[0059] FIG. 17A is a graphical representation of the current output
for the sensor of example no.12 over a range of voltages from -0.6V
to 1V.
[0060] FIG. 17B is a graphical representation of the current output
for the sensor of example no.12 in relation to SO.sub.2
concentrations from 0 to 2%.
[0061] FIG. 18A is a graphical representation of the current output
for the sensor of example no.13 over a range of voltages from -0.6V
to 1V.
[0062] FIG. 18B is a graphical representation of the current output
for the sensor of example no.13 in relation to SO.sub.2
concentrations from 0 to 2%.
[0063] FIG. 19A is a graphical representation of the current output
for the sensor of example no. 14 over a range of voltages from
-0.6V to 1V.
[0064] FIG. 19B is a graphical representation of the current output
for the sensor of example no. 14 in relation to SO.sub.2
concentrations from 0 to 2%.
[0065] FIG. 20A is a graphical representation of the current output
for the sensor of example no.15 over a range of voltages from -0.6V
to 1V.
[0066] FIG. 20B is a graphical representation of the current output
for the sensor of example no.15 in relation to SO.sub.2
concentrations from 0 to 2%.
[0067] FIG. 21 is a flow chart of the process used to prepare the
sensors of examples nos. 16 and 17.
[0068] FIG. 22 is a schematic illustration of a further preferred
sensor configuration, example no. 16, in accordance with the
present invention, employing a substrate, working electrode,
counter electrode, reference electrode, and temperature
detector.
[0069] FIG. 23 is a schematic illustration of a further preferred
sensor configuration, example no. 17, in accordance with the
present invention, employing a substrate, working electrode,
counter electrode, reference electrode, and temperature
detector.
[0070] FIG. 24 is a graphical representation of the current output
for the sensor of example no.16 over a range of voltages from -0.6V
to 1V.
[0071] FIG. 25 is a graphical representation of the current output
for the sensor of example no.17 over a range of voltages from -0.6V
to 1V.
[0072] FIG. 26 is a schematic representation of a preferred heater
configuration, in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0073] The present invention is directed to a thick film
electrochemical sensor device that is capable of being used to
detect or monitor sulfur dioxide emission levels such as in the
environment near the tops of industrial smokestacks.
[0074] The present invention is directed to the fabrication and use
of a chip-like thick film electrochemical micro-sensor device with
three electrodes (working, reference, and counter electrodes),
wherein the electrodes are contacted with an electrolyte,
preferably an ion conductive resin or membrane, and optionally
including a temperature detector, and a heater. The overall size of
the micro-sensor device is preferably on the order of but not
limited to about 0.5 in.sup.2 to about 2.25 in.sup.2, preferably
about 1 in.sup.2. This sensor is heat-resistant and, therefore, is
able to be placed within a smokestack, near the top, to detect and
measure sulfur dioxide emissions.
[0075] The sensor is preferably made using a thick film technique,
including deposition of multiple electrodes on a substrate.
Electrochemical sensors and thick film techniques for their
fabrication are discussed in U.S. Pat. No. 4,571,292 to C. C. Liu
et al, and U.S. Pat. No. 4,655,880 to C. C. Liu, which patents are
incorporated by reference as if fully written out below.
[0076] The substrate may be formed of plastic, glass, ceramic,
alumina, quartz, or any other material that preferably is inert
relative to the material from which the electrodes are formed and
the material into which the sensor is intended to be placed for
use. Preferably the substrate is an alumina ceramic material. Other
suitable ceramics include aluminum nitride, beryllia, silicon
carbide, silicon nitride, and the like.
[0077] The multiple electrodes include at least one each of a
working electrode, a reference electrode, and a counter electrode.
Preferably, the working electrode and the counter electrode are
formed of the same material, although this is not a requirement.
The material is preferably inert relative to the substrate and the
electrolyte as well as to sulfur dioxide. Examples of materials
suitable for the working electrode and the counter electrode
include but are not limited to gold, platinum, palladium, silver,
silver-silver chloride, and carbon. A preferred material is
platinum. The platinum is applied to the substrate in the form of a
platinum ink, which is commercially available, or can be made using
finely dispersed metal particles, solvent, and a binder.
[0078] Specific examples of suitable materials to form the
reference electrode are silver-silver chloride and mercury-mercuric
chloride (Calomel). Silver-silver chloride is preferred. The silver
is applied to the substrate in the form of a silver ink, which is
commercially available, or can be made using finely dispersed metal
particles, solvent, and a binder. As described in further detail
herein, the silver is exposed to chloride solution to produce the
silver-silver chloride electrode electrochemically. The
silver-silver chloride electrode can also be formed using a thick
film silver chloride ink, which is commercially available, or can
be made using finely dispersed metal particles, solvent, and a
binder.
[0079] The sensor device of the present invention may optionally
further include a temperature detector, which preferably comprises
platinum. The platinum is applied to the substrate in the form of a
platinum ink, similarly to the working and counter electrodes.
[0080] As a further option, the sensor device of the present
invention may include a heater. The heater can be printed onto the
opposite side of the ceramic substrate from the electrodes, using a
conductive material, for example, a serpentine platinum ink
pattern, such as the design shown in FIG. 25, to regulate the
temperature at which the micro-sensor will operate. Other suitable
materials include for example carbon, gold, and other metals. Since
the efficiency and specificity of the micro-sensor device is
temperature dependent, it is desirable to maintain the sensor at a
constant temperature. This is especially useful when the sensor is
employed in an industrial setting to detect or monitor sulfur
dioxide emissions within a smokestack. The sensor is heated to a
temperature greater than the internal temperature of the
smokestack, and maintained at a constant level, to avoid the need
to recalibrate the sensor at various temperatures.
[0081] The electrodes of the sensor device of the present invention
may include a connect portion and a sensing portion. The sensing
portion of the electrode is exposed to the environment, and is in
contact with an electrolyte, preferably an ion conductive material.
The sensing portion finctions to detect the target species, namely
sulfur dioxide. The connect portion of the electrode connects the
electrode to an electrical circuit, and is protected from the
environment by an insulator. The insulator used to protect the
connect portion of the electrodes of the present invention is
preferably glass, or glass-containing dielectric materials, and is
applied in the form of an insulating ink. Other suitable insulators
include but are not limited to polymeric insulating materials and
oxide-based insulating materials. In a preferred embodiment, wires
are soldered to the connect portion of the electrodes using indium
solder. The wires and the solder are then covered with a silicone
paste.
[0082] After the sensor device is formed, the electrolyte is
applied to the sensing portion of the electrodes. The electrolyte
preferably comprises a Nafion.TM. ion conductive material membrane
or resin, but may also be other polymeric ionic conductive
materials. The electrolyte resin or membrane is allowed to dry,
but, during testing and use, the sensor device must be kept moist.
If the sensor device is used to detect sulfur dioxide in gas phase
emissions, the sensor electrolyte must be wetted periodically, such
as being sprayed or misted with water.
[0083] According to the invention, 17 different sensor designs were
drawn on AUTO-CAD.TM., a computer drafting program. Then, through a
thick film process, which is similar to the silk screening process,
silver, platinum, and insulating precursor inks were printed onto
alumina ceramic substrates to form the three electrodes and the
temperature detector. The silver was treated with chloride to form
silver-silver chloride, the material used for the reference
electrode, and platinum was used for the temperature detector, the
counter and working electrodes, and optionally the heater. The
micro-sensors were heated to solidify the components, the wires
were soldered to the contacts, and silicone paste was applied.
Finally, the sensors were tested by exposure to sulfur dioxide
concentrations of from 0 to about 2% in air.
[0084] For these thick film electrochemical sensors with a three
electrode configuration, the Gibbs free energy calculated from the
oxidation-reduction or (redox) reaction of the substance being
detected at the working electrode was used to determine the applied
potential voltage needed to enable the specific redox reaction to
occur. Then the corresponding current produced was measured, and
the current was used to quantify the sulfur dioxide levels.
[0085] In the fabrication process of sensor examples 1-15, a
Rubylith.TM. cutter was used to draw and cut enlarged designs for
each layer of the sensor on red Rubylith.TM. sheets. Afterwards,
tweezers were used to lift away the areas representing where the
chemicals for each layer should be applied. Next, the image was
transferred with a camera that reduces the designs onto
photosensitive glass plates. The plates were then developed, and
used to transfer the design onto photosensitive emulsion sheets.
These sheets were fixed onto stainless steel mesh screens and
block-out material was spread on the areas of the screen where the
chemicals should not go through. These mesh screens were the
templates for the thick film process.
[0086] The screens were loaded into a pneumatic machine, the
thick-film printer, to "silk-screen" the silver, the platinum, and
the insulator precursor ink individually onto the alumina ceramic
substrates. After the thick film process, the ceramic substrates
were placed into a drying oven and heated to remove the solvent. A
preferable temperature is approximately 100.degree. C., although a
range of temperatures may be used. Next, the substrates were fired
in a furnace to solidify the inks onto the substrates. A suitable
temperature is approximately 850.degree. C., although a range of
temperatures may be used. Afterwards, the substrates were diced
using a diamond saw into individual devices. Finally, a Nafion.TM.
resin or membrane was applied manually onto the sensing portion of
each electrode to serve as an electrolyte.
Specific Embodiments of the Inventions
EXAMPLES 1-15.
[0087] Part I--Materials For Preparing Sensors
1 AUTO-CAD Computer Program drying oven Rubylith .TM. cutter
furnace red Rubylith .TM. sheets diamond saw transparencies
scissors & scalpel & wire strippers tweezers indium box
camera 0.75 M Flux photosensitive plates wires (black, red, and
green) developer, fix, and wash soldering pen (can heat up
(chemical solutions used to to 999.degree. C.) and holder develop
the photosensitive plates) emulsion sheets silicone paste stainless
steel mesh screens 0.1 M HCI solution ultraviolet light
Bi-Potentiostat & connecting circuit wires paper towels 1000-ml
beaker block-out platinum mesh screen thick film printer (a
pneumatic machine) eraser on mechanical pencils silver precursor
ink de-ionized water platinum precursor ink Nafion .TM. (5%
fluorinated ion-conductive resin) alumina ceramic substrates timer
blue insulator precursor ink pipet and pipet tips
[0088] Part II--Materials For Testing Of The Sensors
[0089] 15 developed thick film electrochemical micro-sensors for
sulfur dioxide wires (three different colors)
[0090] approximately 2% sulfur dioxide gas in air
[0091] gas mixer
[0092] air
[0093] tubes for transferring gases
[0094] potentiostat
[0095] ring stand
[0096] aspirating tube
[0097] 0.005 M potassium nitrate solution
[0098] aspirating Erlenmeyer flask
[0099] de-ionized water
[0100] rubber stoppers (with tube to let stream of gas through)
[0101] masking tape & scissors
[0102] computer & printer
[0103] CHI660A chemistry workstation computer program
[0104] Slide Write computer program
[0105] 50 ml & 100 ml beakers & cardboard
[0106] tiny screw driver (-)
[0107] timer
Experimental Procedures
[0108] The thick film electrochemical sensors according to the
invention were fabricated according to the procedure below.
[0109] The drawings contain the design of 15 thick film
electrochemical micro-sensors prepared and tested in accordance
with the invention, separated into Set A (FIG. 1A), and Set B (FIG.
1B).
[0110] FIGS. 6A-20B demonstrate the results obtained from the test
runs for all 15 sensors. Included for each sensor are graphs which
sweep the voltages from -0.6V to 1V for the corresponding current
outputs. Further, for each sensor there is a scatter plot graph of
the data, the fitted line and the curve fitting coefficients. Part
I: Design & Development
[0111] 1.) Utilize the computer program AUTO-CAD.
[0112] 2.) Create and draw 15 designs on AUTO-CAD in full detail
including dimensions in two sets; Set A (3.75".times.3.75") with 11
designs and 1 repetition and Set B (3".times.3") with 4 designs.
Note: Make separate color layers for each chemical. Color the
reference electrode layer pink for silver; temperature detector,
the counter, and the working electrode aqua for platinum; the
insulator dark blue for insulating ink, and the border of each
sensor black to mark the size of the alumina ceramic substrate.
[0113] 3.) Create alignment marks in the corners of the designs for
each set.
[0114] 4.) Draw dicing marks at the corners as well as at the two
ends of each line which need to be diced or cut.
[0115] 5.) Convert the AUTO-CAD file into the type of programming
form used by the Rubylith.TM. cutter.
[0116] 6.) Run the Rubylith.TM. cutter to enlarge and to cut the
designs for each layer of Set A on red Rubylith.TM. sheets.
[0117] 7.) Use tweezers to lift away the areas of the Rubylith.TM.
representing where the chemicals need to be applied.
[0118] 8.) Prepare to develop sensor materials in a dark room.
[0119] 9.) Move into a separate room containing the green light
board.
[0120] 10.) Tape the sheet of Rubylith.TM. for Set A platinum onto
the green light board.
[0121] 11.) Position the camera and open the lens to test take the
image.
[0122] 12.) Reduce the size of the Rubylith.TM. image and transfer
the image onto a photosensitive plate using the camera which
exposes the plates to the green light from the light board for
about 1 minute 30 seconds to about 2 minutes.
[0123] 13.) Take the plate to the dark room. Note: Do not expose it
to any light.
[0124] 14.) Develop the plates by placing them for 4 minutes in the
developer, 30 seconds in the fix, and 4 minutes in the wash. Note:
rock the trays containing the solutions back and forth throughout
the development to keep fresh solutions on the plates.
[0125] 15.) Wash the plates under running water and dry them.
[0126] 16.) Repeat steps 10-15 for all of the layers in Set A
(silver and insulating ink).
[0127] 17.) Move to another dark room containing the ultraviolet
light source.
[0128] 18.) Cut out three sheets of emulsion about the size of the
photosensitive plates.
[0129] 19.) Place each plate over an emulsion sheet under the
ultraviolet light source for half an hour or more.
[0130] 20.) Place the emulsion sheets individually in the developer
for approximately 4 minutes.
[0131] 21.) Run the sheets under hot water to remove the areas
where the chemicals need to be applied.
[0132] 22.) Place each emulsion sheet the red sticky side up.
[0133] 23.) Press the stainless steel mesh screen on top of each
emulsion sheet and press a paper towel on top of it to absorb the
extra emulsion.
[0134] 24.) Leave the stainless steel mesh screens to dry.
[0135] 25.) Apply block-out to areas where the chemicals should not
go through.
[0136] 26.) Allow these mesh screens to dry again to become the
templates for the thick film process.
[0137] 27.) For Set B, print out the designs onto transparencies
with one transparency for each layer.
[0138] 28.) Repeat steps 17-26, except this time using the
transparencies rather than the developed photosensitive plates.
[0139] 29.) Load the template for the reference electrodes for Set
A into a pneumatic machine, thick-film printer.
[0140] 30.) Bring the template into contact with the substrate and
align.
[0141] 31.) Spread the silver precursor ink onto the loaded
template.
[0142] 32.) Apply the reference electrode precursor ink by
"silkscreening" the silver precursor ink onto the substrate.
[0143] 33.) Repeat step 32.
[0144] 34.) Repeat this process for as many sensors as desired.
[0145] 35.) Apply the working electrode precursor ink, counter
electrode precursor ink, and temperature detector precursor ink by
loading the appropriate template and "silkscreening" the platinum
precursor ink onto the substrate, as in steps 29-34.
[0146] 35.) Apply the insulating precursorink, using the technique
described above in steps 29-34.
[0147] 36.) Place the substrate onto which the electrode precursors
have been applied into a drying oven and heat at about 100.degree.
C. for about 1/2 hour to remove the solvent.
[0148] 37.) Fire the substrate in a furnace at about 850.degree. C.
for about 1 hour to cure the electrode precursors and solidify the
sensor device.
[0149] 38.) Dice the sensor device along the dicing marks using a
diamond saw.
[0150] 39.) Repeat steps 29-38 for Set B.
[0151] 40.) Break the sensors along the diced lines into separate
pieces.
[0152] 41.) Cut the indium into small pieces.
[0153] 42.) Cut wires red, green, and black into 15 cm lengths with
one of each for each sensor device.
[0154] 43.) Expose about 0.5 cm of bare wire on either end.
[0155] 44.) Spread the flux onto the patches of the silver and
platinum where the wires need to be soldered.
[0156] 45.) Heat the soldering pen up to approximately 535.degree.
C.
[0157] 46.) Use the pen to melt the indium onto the patches on the
sensors that have been covered by flux.
[0158] 47.) Dip one end of each wire into the flux and cover with
indium using the pen.
[0159] 48.) Once again, use the pen to melt the indium on the wire
with the indium on the sensors.
[0160] 49.) Repeat steps 44-48 for each sensor device.
[0161] 50.) Clean each sensor using methanol and then dry.
[0162] 51.) Apply silicone paste to cover the bare wires and the
indium.
[0163] 52.) Allow the silicone paste to solidify overnight.
[0164] 53.) Twist the free ends of the wires connected to the
silver electrodes of three to five micro-sensors together, so they
can be exposed to chloride simultaneously.
[0165] 54.) Clean the silver surface using a mechanical pencil
eraser.
[0166] 55.) Fill a 100-ml beaker with 800 ml of the chloride
solution.
[0167] 56.) Connect a platinum screen to the negative side of the
potentiostat.
[0168] 57.) Connect the twisted wires to the positive.
[0169] 58.) Place both the screen and the sensors into the beaker
without allowing them to touch one another.
[0170] 59.) Turn the voltage to 0.5V.
[0171] 60.) Clean the silver surfaces first by turning the power up
for 5 seconds and down for 5 seconds three times.
[0172] 61.) Allow the chloride to react with the silver to form
silver-silver-chloride by leaving the power on for 2 minutes.
[0173] 62.) Repeat steps 57-61 for all of the sensors.
[0174] 63.) Rinse the sensors using warm water and de-ionized
water.
[0175] 64.) Place them on paper towels to dry.
[0176] 65.) Apply Nafion.TM. resin or membrane, which will serve as
the electrolyte, manually onto the sensing portion of each
electrode individually using the pipet and a clean pipet tip.
[0177] One preferred sensor design is shown in FIG. 2A. The
micro-sensor device shown is the example number 3 from FIG. 1A. The
temperature detector 54 and contacts 60, 61 are platinum. The
counter electrode 55 and working electrode 64, with gap 57 between
them, are also platinum. The reference electrode 58 is silver
silver-chloride. The contacts 62, 63, 65 provide sites to connect
wires. A portion of the electrodes is covered with glass insulator
59.
[0178] A second preferred sensor design is shown in FIG. 2B. The
micro-sensor device shown is the example number 7 from FIG. 1A. The
temperature detector 99 and contacts 100, 101 are platinum. The
counter electrode 98 and working electrode 102, with gap 107
between them, are also platinum. The reference electrode 103 is
silver silver-chloride. The contacts 104, 105, 106 provide sites to
connect wires. A portion of the electrodes is covered with glass
insulator 108.
[0179] The 15 thick film electrochemical micro-sensors fabricated
as described above were tested in one of the device shown in FIGS.
3 and 4. The setup in FIG. 3 was used to test the sensors of
example nos. 1-11 and 14-15. The gas entered the device through a
tube 23 which was connected to a plastic tube 21 that was fitted
with a rubber stopper 22, and passed into an Erlenmeyer flask 20
containing deionized water. Another tube 24 transported the gas
from the flask 20 to a glass tube 25, supported by a clamp 28 and a
ring stand 29. Within the glass tube 25 was suspended the
micro-sensor 26 to be tested. Attached to the micro-sensor device
were three electrical wires 32, 33, 34 that extended out of the
glass tube 25 through a rubber stopper 30 and were connected to a
potentiostat 35.
[0180] The sensors of example nos. 12 and 13 were tested using the
setup in FIG. 4. The gas entered through a tube 48 into a beaker
40, fitted with a cardboard covering 45, and containing deionized
water. The tube 48 passed through a rubber stopper 46. The beaker
40 contained a metal weight 41. Suspended above the surface of the
deionized water was the sensor to be tested 44. Connected to the
micro-sensor device were three electrical wires 49, 50, 51, which
passed through the covering 45 and were connected to a potentiostat
35.
[0181] Part II: Testing
[0182] 1.) Wait 1 day after application of Nafion.TM. resin or
membrane.
[0183] 2.) Connect the gas tubes of the tanks of sulfur dioxide and
air to the gas mixers.
[0184] 3.) Connect another gas tube from the mixer to the plastic
tube on the rubber stopper.
[0185] 4.) Fill the aspirating Erlenmeyer flask with approximately
200 ml of de-ionized water, and plug the rubber stopper onto the
top of the flask. (FIG. 3).
[0186] 5.) Connect another tube to the opening on the side of the
flask and attach the other end to the skinny end of the glass
tube.
[0187] 6.) Cut three approximately 125 cm wires of different colors
and connect one to each of the circuit wires from the potentiostat,
which is in turn connected to a computer with an interface.
[0188] 7.) Extend the wires to the hold while taping the wires onto
the floor.
[0189] 8.) Initiate the computer.
[0190] 9.) Open the CHI660A Chemistry Workstation computer
program.
[0191] 10.) Soak the electrodes of a micro-sensor in the sodium
nitrate solution for 30 minutes.
[0192] 11.) Place the wires on the micro-sensors through the
plastic tube of another rubber stopper.
[0193] 12.) Connect the wire of the counter electrode with the
negative wire, the working electrode with the positive, and the
reference electrode with the reference electrode.
[0194] 13.) Make sure the three bare portions of the wires are not
touching one another.
[0195] 14.) Open the valves of the two gas tanks both to 30
psi.
[0196] 15.) Turn on the gas mixer and adjust the mixing to 400 for
air and 0 for sulfur dioxide gas and turn on outlet 1 for air gas
(0% sulfur dioxide).
[0197] 16.) Let the gas bubble through the water for 5 minutes to
stabilize.
[0198] 17.) While the gas is bubbling, set the print setup to
landscape and set the program to the following requirements:
[0199] a.) Initial Voltage to 0V
[0200] b.) High Voltage to 1V
[0201] c.) Low Voltage to -0.6V
[0202] d.) Initial Direction to Negative
[0203] e.) Scan Rate to 0.1 V/sec
[0204] f.) Segments to 3
[0205] g.) Sample Interval to 0.001V
[0206] h.) Quiet Time to 0 sec
[0207] i.) and Sensitivity to 0.001 A/V.
[0208] 18.) Click run experiment.
[0209] 19.) Save the test run.
[0210] 20.) Turn off outlet 1 and adjust the mixing air gas to 200
and sulfur dioxide gas to 200 before turning both 1 and 3 (1%
sulfur dioxide).
[0211] 21.) Let the gas mixture bubble through the water for 5
minutes while printing the graph of the test run with inverted x-
and y-axes and copying the data for segment 2 at 0.4V, 0.5 V, 0.6V,
0.7V, and 0.8V. Also, during this time make sure the requirements
from the above are set.
[0212] 22.) Repeat steps 18-19.
[0213] 23.) Repeat steps 20-22 with 400 for sulfur dioxide gas and
0 for air and only turning on outlet 3 (2% sulfur dioxide).
[0214] 24.) Turn off the gas mixer and detach the mixers before
removing them from the glass tube.
[0215] 25.) Rinse the micro-sensor as well as the emptied
aspirating Erlenmeyer flask with de-ionized water.
[0216] 26.) Turn off the potentiostat and the valve of the two gas
tanks.
[0217] 27.) Open the Slide Write computer program.
[0218] 28.) Insert the data with 0, 1, and 2 repeated 5 times in
the x-column and the current values in micro-amperes for 0.4V in
graph A, 0.5V in B, 0.6V in C, 0.7V in D, and 0.8V in E.
[0219] 29.) Set the graph type to scatter, the graph fitting to
linear, the legends to right, the x-axis to label and to from 0-2
with 2 divisions, and the y-axis to values appropriate for the
obtained data.
[0220] 30.) Click redraw chart.
[0221] 31.) Click on statistics to determine the curve fitting
coefficient, r.
[0222] 32.) Label each graph by its voltage and curve fitting
coefficient in the legends.
[0223] 33.) Select save and print.
[0224] 34.) Repeat steps 8-33 for the other sensors except 12 and
13, which will not fit in the glass tube.
[0225] 35.) Repeat steps 8-33 for sensors 12 and 13, but use the
setup of FIG. 4:
[0226] a.) Place a beaker in the hold.
[0227] b.) Place the metal weight as the bottom and fill the beaker
with enough de-ionized water to cover the metal block.
[0228] c.) Cut a cardboard cover for the beaker with a hole for
inserting the plastic tube from the rubber stopper.
[0229] d.) Place the cardboard cover over the beaker and insert the
plastic tube from the rubber stopper connected to the gas
tubes.
[0230] e.) Dangle the sensors in the beaker without touching the
water.
[0231] The thick film sensors of the present invention operate
based on oxidation and reduction reactions. The Gibbs free energy
calculated from the oxidation-reduction (or redox) reaction of the
substance being detected at the working electrode was used to
determine the applied potential voltage needed to enable the
specific redox reaction to occur. The range from -0.6V to 1 V was
chosen because that was the range for which a little oxidation
occurred as well as little reduction Representative chemical
equations are as follows: 1
[0232] When the voltages are applied, the way that the electrodes
operate is shown in the diagram of FIG. 5. As the sulfur dioxide
contacts the working electrode, an electrochemical reaction occurs,
resulting in higher current. The counter electrode is driven by the
potentiostat to maintain the potential difference between the
working electrode and the reference electrode. A measurement of
current at the working electrode is related to the concentration of
the sulfur dioxide. The sensor can therefore be calibrated at any
given operating temperature by known techniques. Test results for
Examples 1-15 are shown in the graphs of FIGS. 6A through 20B.
[0233] Three main types of three-electrode arrangements were
prepared and tested in the initial phase. Five of the fifteen
examples employ a rectangular electrodes arrangement. These include
sensor example numbers 1, 5, 9, 10, and 13. Another type of
arrangement was prepared, in which reference electrode is a circle.
Seven of the fifteen examples (3, 4, 8, 11, 12, 14, and 15) have
this configuration, but these can be divided into two branches.
Four of the seven examples, including numbers 3, 11, 12, and 14,
have the counter electrodes and the working electrode almost
completely encircling the reference electrode, while the other
three have the counter electrode and working electrode both being
half circles. The last type of configuration utilizes an elliptical
reference electrode with a working electrode and a counter
electrode that are almost full ellipses themselves. Sensor designs
with this configuration include examples 2, 6, and 7. All of these
various configurations were tested to determine whether the shape
and arrangement of the electrodes would have an effect on how the
sensors operate to detect sulfur dioxide.
[0234] Within each type of electrode arrangement, the sizes of the
electrodes as well as of the sensors themselves vary from sensor
design to sensor design. This made it possible to study the effect
of the size of the electrodes on sensitivity, as well as on the
efficiency of the sensors.
[0235] Another variable investigated was the distance of the gaps
between the working and counter electrodes. Within each type of
configuration, the gaps between the working and counter electrodes
also varied due to changes in sizes. By varying the gap distances,
the effect that these gaps have on the sensors' sensitivity and
efficiency was observed. It was found that optimum sensitivity and
efficiency was observed when the gap size was less than or equal to
about 0.2 inches, and preferably less than or equal to about 0.1
inches. More preferred is a gap size of about 0.04 to about 0.05
inches, as exemplified in the sensors of examples 3 and 7. Care
should be taken, however, that the inks do not bleed into each
other during fabrication, as this will cause a short circuit in
operation.
[0236] Although the designs of the micro-sensors were created with
various differences in the size and the shape of the electrodes, in
order to compare the effect that these alterations have on the
accuracy of the sensors, the materials used to form the electrodes,
platinum and silver-silver-chloride, were not altered. Based on
their individual characteristics, it was considered that these two
materials were the preferred choices. Platinum is an inert
conductor as well as a noble transition metal, which does not react
readily with sulfur dioxide, and it is also highly reversible. On
the other hand, silver/silver-chloride is also reversible with a
low EMF, making it an optimum choice for the reference electrode.
Although hydrogen is also a good choice for the reference
electrode, it would be harder and more expensive to retain a
gaseous hydrogen electrode, especially at high temperatures.
Therefore, the materials of platinum for the working electrode and
counter electrodes, and of silver-silver-chloride for the reference
electrode are preferred.
[0237] The first of the configurations discussed above was the
rectangular configuration that was used for the sensors of examples
1, 5, 9, 10, and 13 (which will be discussed later). In the
rectangular configuration, the working electrode and the counter
electrode are placed with the reference electrode interposed
between them. Because of this, the gap between the working and
counter electrodes is approximately 0.2 inches or more, greater
than that of other configurations, and there are no regions where
the working and counter electrodes are adjacent to one another.
This increased gap size, as well as the lack of adjacency between
the working and counter electrodes, leads to an increased
resistance when the electrons are transferred from the counter to
the working electrode. The increased resistance, in turn, causes
the sensors of this configuration to have a lower current output or
a lower sensitivity. This can be seen from the graphs of the test
runs for these five sensors' current output, which are all less
than 50 .mu.A, as shown in FIGS. 6B, 10B, 14B, 15B, and 18B.
[0238] The sizes of the electrodes varied on the micro-sensors that
had rectangular configuration. However, as the sizes increased, the
gaps between the working and the counter electrodes also increased.
Because of this and the fact that, for these sensors, the
difference in the gap distance is larger, making the gap distance
have a greater effect on the sensors' current output, the current
decreased as the gaps increased along with the sizes. This can be
seen in comparing the graphs of the smaller sensor of example 9
(FIG. 14B) to that of the larger sensor of example 1 (FIG. 6B).
Furthermore, because of the greater resistance created by the
increased gap, the larger sensors also tend to have less accurate
readings, as can be seen by the curve fitting coefficients which
are further away from 1 (a perfect fit). Because examples 9 and 10
had very similar gap distances, but example 9 had larger
electrodes, the effect created by size of the electrodes alone was
able to be derived as well. In comparing these two, it was seen
that example 9 (FIG. 14B) had higher current output than example 10
(FIG. 15B). From this, it was derived that, when the size of the
electrodes increase without enlarging the gap, the current output
increases as well.
[0239] The second type of configuration utilized was the
semi-circle configuration used for the sensors of examples 4, 8,
and 15. For these sensors, although the working and the counter
electrodes are further apart at the position where the reference
electrode circle has its horizontal diameter, the working and
counter electrodes are closer to one another at the other portions.
Moreover, at the lowest point of the electrodes, these two
electrodes are adjacent to one another. Therefore, when the sizes
are smaller, the output current is greater (FIGS. 9B, 13B, and 20B)
in comparison to the rectangular configurations. However, when the
sizes are larger the working and counter electrodes are further
apart than in the rectangular configuration for a larger portion of
the sensor, and the output current just about as small as in the
rectangular configuration.
[0240] The sizes were varied within these "semi-circle" sensors as
well. Once again, the gap distance increased more significantly
than the size as the circular reference electrode increased. This
again caused the current output to decrease significantly, as can
be seen by comparing the graphs of example 4 (FIG. 9B) with
currents in the 10-20 .mu.A range, and example 8 (FIG. 13B) with
currents in the 100-200 .mu.A range. In this group, because of the
configuration, it was not possible under the experimental
conditions to enlarge the whole sensor without increasing the gap
distance. Therefore, it was not feasible to compare the effects of
just variations in the sizes.
[0241] The third configuration utilized concentric circles
electrodes, which included examples 3, 11, 12 (which will be
discussed later), and 14 (FIGS. 8B, 16B, 17B and 19B). This
configuration had very small gaps between the working and the
counter electrodes because, for these sensors, the counter
electrode and the working electrode run adjacent to one another for
the most part. Another difference is that the size of the working
and counter electrodes on the concentric circles sensors are almost
double the size of the working and counter electrodes on the
semi-circles sensors because, on the concentric circles sensors,
the circles are almost full. Due to these two main factors, these
sensors have very high current outputs which are thousands of
.mu.A, and correspondingly have high sensitivity.
[0242] Within this group of sensors, there was also size variation.
However, in increasing the sizes of the sensors, the gap was
enlarged. Therefore, because of the enlarged gap size and the fact
that the difference in the gap size for these was greater than that
of the sensor size, the current output of the larger sensor of
example 14 was less than that of the smaller sensor of example 3.
However, because the difference in electrode size is so much more
significant than that of the gap for sensors of examples 11 and 3,
the current of the sensor of example 11 actually decreased in
comparison to that of the sensor of example 3, despite the
decreased gap size. Therefore, as demonstrated, increased electrode
size does in fact lead to higher sensitivity.
[0243] Another type of configuration utilized was that of
concentric ellipses, which includes the sensors of examples 2, 6,
and 7. In this type of configuration, the working and the counter
electrodes run adjacent to one another for the most part. However,
when compared to the circular electrode designs, because these
ellipses are elongated circles, the gap is slightly larger at the
bottoms of these ellipses. Along the sides, the gaps are slightly
smaller. Furthermore, the electrodes of the sensors with the
concentric ellipses configuration are larger than those of the
concentric circles, because they are longer vertically. Because of
these factors, the range of the current output is a couple thousand
microamperes higher than that of concentric circles, and this
design therefore provides a higher sensitivity.
[0244] Within this group of sensors, the sizes were varied as well.
However, in this set, the gap tended to change less than it did for
other configurations. Therefore, for this group, the changes in the
gaps are minor compared to the variations in the electrodes' sizes.
Because of this, the effects of electrode size on the sensitivity
and current output could be seen. By comparing the graphs of the
three sensors' test runs and line fitting graphs, (FIGS. 7B, 11B,
and 12B), it was observed that as the electrode size increased the
current output increased significantly as well.
[0245] Finally, comparing the four configurations it was determined
that the concentric designs were better. These configurations have
the working and the counter electrodes adjacent to one another for
a longer distance with smaller gaps. Furthermore, the surface area
of the working and counter electrodes is larger in the concentric
designs than other designs. These factors make the current outputs
higher and the curve fitting coefficients somewhat closer to 1. The
higher current outputs of the concentric designs make them not only
more sensitive, but also more efficient, because the higher current
outputs are easier to detect.
[0246] Two sensors, of examples 12 and 13, were excluded from the
above discussion, as these sensors were tested with a different
setup (FIG. 4). In this other setup, the gas was not able to flow
as freely through the region where the sensors were placed.
Instead, the gas bounced back and forth within the beaker.
Furthermore, the gas, instead of travelling through a gas tube
first, was directly exposed to the sensors. Therefore, the test
results were significantly different from the others. Although they
fit the trends concerning size and gap, the coefficient r was very
far from 1, and the line intersected and even had negative slopes
(FIGS. 17B and 18B). Contributing factors could be either that the
sensors were malfunctioning, or that the setup was not appropriate
for this kind of testing.
[0247] As shown by the above examples, the current
output/sensitivity did increase as the gap between working and
counter electrodes decreased, as well as where the length of the
working and counter electrodes were adjacent to one another
increased. In the optimum configuration, it is preferable that the
working and the counter electrodes are adjacent to one another for
at least 90 percent of their exposed length, but more especially,
it is preferable that the working and the counter electrodes are
adjacent to one another for at least 90 percent of their entire
length, having a gap therebetween of up to about 0.2 inches. There
does exist a linear relationship between current outputs and
concentrations, as can be seen by the fact that, for most of the
sensors which functioned correctly, the coefficient r was
relatively close to 1. Finally, the current output increased as
electrode size increased when the gap size did not increase at the
same time. If the gaps enlarged significantly as well, the current
output actually decreased.
[0248] Part III: Design and Testing of Examples 16-17
[0249] Based on the above results, two additional sensor
configurations were designed and prepared as shown on the flow
chart of FIG. 21. Micro-sensor examples 16 and 17 were also
prepared using a thick film technique, similar to examples 1-15,
except that for examples 16 and 17, the designs were printed
directly onto transparencies. The transparencies were then used to
transfer the designs onto emulsion sheets.
[0250] The configuration of sensor example 16 is shown in FIG. 22.
The counter electrode 68 is interposed between the reference
electrode 69 and the working electrode 70, with a gap 77 between
the working and the counter electrodes. The reference electrode is
disposed outwardly to the working and the counter electrodes. All
three of these electrodes and the temperature detector 71 are
applied to one side of the substrate 72. The contacts 73, 74, 75,
76, and 78 provide a connect portion of the sensor to which wires
are soldered and covered with silicone paste. Insulator 79 protects
the connect portion of the sensor device from the environment.
[0251] The configuration of sensor example 17 is shown in FIG. 23.
The counter electrode 81 and the working electrode 80 are
interdigitated, with a gap 87 between them, and with the reference
electrode 82 disposed outwardly to them. All three of these
electrodes and the temperature detector 83 are applied to one side
of the substrate 84. The contacts 85, 86, 88, 89, and 90 provide a
connect portion of the sensor to which wires are soldered and
covered with silicone paste. Insulator 91 protects the connect
portion of the sensor device from the environment.
[0252] Examples 16 and 17 were tested in accordance with the
procedures used for example 1. Typical current outputs for sensor
example 17 at various voltages and concentrations of sulfur dioxide
are shown in Table 1. Graphical representations of the current
output across a range of voltages from -0.6 to 1.0 for example 16
and 17 are shown in FIGS. 24 and 25, respectively.
2TABLE I Microamperes of Current Output from Sensor Example 17 0%
SO.sub.2 0.5% SO.sub.2 1.0% SO.sub.2 1.5% SO.sub.2 2.0% SO.sub.2
0.4 V 81.55 91.96 116.5 163.9 220.7 0.5 V 106.1 128.7 170.4 247.3
339.9 0.6 V 138.8 167.5 221.3 324.8 451.9 0.7 V 169.9 201.1 264.0
388.4 545.7 0.8 V 196.1 229.4 299.5 441.1 624.8 0.9 V 219.9 254.4
331.3 487.9 692.9
[0253] The two sensor examples 16 and 17 also include a heater of
platinum, printed on the opposite side of the substrate. The design
of the heater is shown in FIG. 26. The heater is a substantially
serpentine pattern of platinum 92 printed on a substrate 93.
Contacts 94 and 95 provide sites to connect wires.
[0254] The temperature detector can be calibrated by measuring the
resistance at various temperatures. The temperature detector can
then be used to test the heater and to correlate the required
voltage inputs necessary to maintain the temperature of interest.
In actual operation, such as in an industrial smokestack, the
heater will operate to maintain the micro-sensor device at a
constant temperature, preferably above the temperature of the
emission gas in contact with the sensor, thereby insuring the
accuracy of the calibration for sulfur dioxide detection or
monitoring.
[0255] Other sensor configurations are possible according to the
present invention, but a more preferred configuration has a portion
of the working and counter electrodes interdigitated with a small
gap between them, on the order of about 0.2 inches or less.
Furthermore, in this embodiment, the reference electrode is
disposed outwardly to the working and the counter electrodes.
Preferably the micro-sensor also includes a mesh screen to protect
it against harmful dust, ashes, or particles, which may disrupt the
accuracy of the measurements. The micro-sensor may be further
adapted to perform an actuating function, such as to trigger the
spraying of a certain basic substance, such as calcium hydroxide,
which will scrub or neutralize the sulfur dioxide.
[0256] Advantageously, the micro-sensor device of the present
invention, prepared using a thick film technique, is relatively
inexpensive to manufacture, install, and operate. For this reason,
it is possible to use two or more sensors and operate them in a
differential mode. In a preferred embodiment, two substantially
identical sensors are used. One sensor is optimized for sulfur
dioxide detection, through the choice of operating temperature,
electrolyte membrane, the use of an electrode catalyst, or by
another method, and the second sensor is adapted to detect
interference from other chemical species, such as being operated at
ambient temperature. The level of sulfur dioxide can then be
determined by subtracting the signal due to the interference from
the signal of the sulfur dioxide detecting sensor. Such a method of
differential operation can overcome the problems of interference
that are known in the art of electrochemical sensors. The method of
this embodiment comprises contacting the emission gas with a first
inventive sensor, measuring the current output of the sensor
adapted to detect sulfur dioxide generating a first signal based on
the current output of the sensor, providing a second inventive
sensor, which has been adapted to detect interference from other
chemical species, contacting the emission gas with the second
sensor, measuring the current output of the second sensor,
generating a second signal, and subtracting the second signal from
the first signal. This signal can then be used to activate a
display device, a recording means, an alarm device, and/or a
compensating means.
[0257] It is demonstrated that the electrochemical micro-sensor
device of the present invention can be used to detect sulfur
dioxide emissions cheaply and quite effectively in various
locations, including the upper part of an industrial smokestack,
where the temperatures are lower. When SO.sub.2 is detected, the
sensor generates a signal that is sent to an indicator, such as an
alarm, or visual display, or to a recorder, making it possible to
study process trends and track emissions over a period of time.
Current generated can be measured by a potentiostat, for example,
or analyzed by computer or another electronic measuring device. The
sensor can generate a signal that is amplified if necessary, and
that triggers an actuator, to actuate a neutralizing apparatus such
as an existing scrubber system only when a predetermined level of
sulfur dioxide is detected, allowing more efficient utilization of
the scrubber system. The sensor could also be used to initiate
diversion of the emission gas when necessary into a cold trap
system or reservoir by opening or closing a valve to allow further
treatment of the emission gas, or to initiate shutdown of the
chemical or physical process producing the emissions.
[0258] It should now be apparent that various embodiments of the
present invention accomplish the object of this invention. It
should be appreciated that the present invention is not limited to
the specific embodiments described above, but includes variations,
modifications, and equivalent embodiments defined by the following
claims.
* * * * *